Known constructions of electromagnetic pick-up devices of the kind mentioned above are arranged so that a measuring tube of non-magnetic steel is provided with a relatively thin layer of an electrically insulating lining made of different insulating materials, e.g., rubber, ceramic material, teflon and other plastics. The electrodes are made of different electrically conductive metals, e.g., stainless steel, tungsten, platinum, hastealloy and the like. Due to abrasion produced by the streaming thickened medium, the lining is significantly abraded, sometimes in the course of less than one year, so that particularly in the case of conduits of large diameters, there is a substantial difference in thickness as compared with the original results. Further, the lining is frequently completely abraded, so that the pick-up device becomes inoperative and unfit for use. Because the material of the electrode has a substantially higher resistance to abrasion than the material of the lining, the electrode remains after the abrasion thereof, and with respect to the portion exposed in the throughflow cross section, vortexes are created and the lining is attacked in the area behind the electrode. For horizontal conduits a sedimentation of solid particles is experienced in the course of passage of thickened liquids so that the rotationally symmetric profile is substantially affected, and the indication produced by the flow indicator is subject to substantial error. For solid particles of a size of 1 mm and of a specific weight of 1.5 kg/dm.sup.3 this error is, for instance, up to 3% when using a pair of electrodes situated at the center of the throughflow cross section.